Which of the following is NOT a form of modulation?

Voltage modulation is not considered a standard form of modulation in the context of communication systems. In telecommunications, modulation refers specifically to techniques used to encode information onto carrier waves by varying certain characteristics of the wave itself. The primary forms of modulation include amplitude modulation (changing the amplitude of the carrier wave), frequency modulation (altering the frequency of the carrier wave), and phase modulation (modifying the phase of the carrier wave).

While voltage modulation may exist in other contexts, particularly in electronic circuits where voltage levels can control output or signal strengths, it does not fit within the established categories of modulation techniques typically used for audio or radio signals. In contrast, amplitude, frequency, and phase modulation are all widely recognized and utilized in various communication systems for transmitting data effectively.